In the automobile industry, recently, there have been an increasing demands for fuel economy as concerns about environmental problems mount. To meet such demands, the automobile industry is researching ways to reduce the weight of car bodies, and is attempting to increase the use of magnesium alloy in automotive parts because of it is lightest in weight among metals which may be practically used. Recently, in particular, it has been research for use in parts demanding very high corrosion resistance, such as the outer housing and structural parts.
However, since magnesium alloy is the most common practical alloy, when fastened together with different metals such as iron and aluminum, electrolytic corrosion is likely to occur in the presence of moisture containing electrolytes. In particular, in the engine space and at the underside of an automobile, electrolytic corrosion is extremely promoted by the action of electrolytes contained in rainwater, melting snow, salt, etc., and problems, that is, loosening, may occur in the fastened parts. Hitherto, as disclosed in Japanese Patent No. 2715758, aluminum washers were insulated by anodic oxidation, or bolts were coated with resin as disclosed in Japanese Patent Publication No. S58-40045.
However, anodic oxidation of aluminum washers is very expensive. In the case of coating bolts with resin, contact adhesion of resin coating films on bolts and durability are insufficient, and the coating film may peel off, resulting in electrolytic corrosion, and improvement in contact adhesion is required.